1. General Field of the Invention
This invention relates to a retroreflectors, to reflector systems, to optical apparatus, to methods of manufacturing reflector systems, to Jamin-type interferometers, to beam splitting blocks and to methods of manufacturing beam splitting blocks.
2. Particular Fields of the Invention and Description of the Prior Art
A first aspect of the invention relates to a retroreflector, and more particularly to a retroreflector having three mutually-orthogonal reflective surfaces arranged around an optical axis. Such retroreflectors are well known and have applications in, for example, vehicle rear reflectors and interferometers. In a typical Michelson-type interferometer, a reference beam and a measuring beam derived from a common beam are projected at right angles to each other and are then reflected back by respective reflectors; the reflected beams are superposed to form an interferogram. By contrast, in a typical Jamin-type interferometer (see, e.g., FIGS. 1 and 3 of patent document US-A-4571082 and FIGS. 16a,b of patent document US-A-5546184), a reference beam and a measuring beam derived from a common beam are projected parallel to each other, but spaced apart, and then back by a single reflector; again, the reflected beams are superposed to form an interferogram. Retroreflectors can usefully be used in such interferometers so that parallelism of the reflected and projected beams can be assured and so that the reflected beam can be offset from the projected beam. However, the use of a single conventional retroreflector for both the reflected and projected beams in a Jamin-type interferometer limits the applications of the interferometer.
A second aspect of the invention relates to a reflector system comprising first and second relatively movable reflectors.
A third aspect of the invention relates to an optical apparatus having an optical axis and a retroreflector.
Fourth and fifth aspects of the invention relates to a method of manufacturing a reflector or reflector system.
Sixth, seventh and eighth aspects of the invention relates to Jamin-type interferometers.
The Michelson-type interferometer mentioned above typically employs polarisation techniques to advantage, but consequently suffers from the disadvantage of the need to align the polarisation directions of the light beam source and polarisation-affecting components in the interferometer. By contrast a Jamin-type interferometer as described above typically does not use polarisation techniques and so does not suffer from the polarisation alignment problem. Indeed, alignment generally of a Jamin-type interferometer is less problematic than a Michelson-type interferometer. A further disadvantage of the typical conventional Jamin-type interferometer is that, because the reference and measuring beams share a common general path and a common reflector, there has been no widespread use of Jamin-type interferometers for displacement measurementxe2x80x94 the Jamin-type interferometer of US-A4571082, for example, is limited to use in conjunction with a refractometer to measure refractive index rather than displacement and the Jamin-type interferometers of US-A-5546184 either are for use with a refractometer or employ a complex reflector arrangement to measure displacement.
The Jamin-type interferometer of the eighth aspect of the invention is concerned with the problem in the type of interferometer shown in FIG. 16a of US-A-5546184 that it is necessary to provide two separate phase-shifting means (the phase-shifting films 135,137 in US-A-5546184) in order to obtain a pair of interferogram beams in approximate phase quadrature.
The seventh and eighth aspects of the invention are concerned more particularly with a Jamin-type interferometer wherein: a beam splitter is arranged to split an incident beam of light into first and second generally-parallel, spaced-apart, projected beams; a reflector system is arranged to reflect the first and second projected beams to produce first and second return beams, respectively, which are spaced apart from and generally parallel to each other and the first and second projected beams; and the beam splitter is arranged to enable the first and second return beams to be superposed to produce at least one interferogram.
The ninth aspect of the invention relates to a method of manufacture of a beam splitting member, comprising the steps of: applying a thin-film, beam-splitting, metal coating to at least part of a surface of the member; and baking the member and coating so as to modify the phase shift produced by the coating.
Such a method is described in Vyskub VG, et al, 1977, Pribory i Tekhnika Éksperimenta, (Moscow Engineering Physics Institute), No 4, pp 210-211. That paper explains that members and coatings had been baked to achieve phase shifts of 90xc2x13xc2x0.
An aim of the first aspect of the present invention is to provide a retroreflector which may increase the applications of a Jamin-type interferometer and which may have other uses.
The retroreflector of the first aspect of the invention is characterised in that the reflective surfaces stop short of the optical axis to provide a central region of the retroreflector which transmits incident light and a peripheral region of the retroreflector which retroreflects incident light. Accordingly, the retroreflector can retroreflect a beam which is incident on the peripheral region (with an offset between the incident beam and the parallel return beam), but can pass a beam which is incident on the central region.
In a first embodiment of the first aspect of the invention, the retroreflector may comprise a body of optical material which provides the reflective surfaces by internal reflection in the body. In this case, the retroreflector is preferably and conveniently in the form of a solid cube corner having a first transmitting surface for incident light and a second transmitting surface which truncates the cube corner. In this case, the first and second transmitting surfaces may be exactly parallel, but for some applications are preferably generally, but not exactly, parallel, so as to reduce the effects of stray reflections. Alternatively, the retroreflector may be in the form of a solid cube corner having a passageway extending therethrough generally in the direction of the optical axis to provide the central region.
In a second embodiment of the first aspect of the invention, the retroreflector may comprise three plane mirror elements arranged around the optical axis, each providing a respective one of the reflective surfaces. In this case, the retroreflector is preferably in the form of a hollow truncated cube corner.
The reflector system of the second aspect of the invention is characterised in that: the first reflector is a retroreflector according to the first aspect of the invention; and the reflectors are arranged so that light which is transmitted through the central region of the first reflector is reflected by the second reflector and transmitted back through the central region of the first reflector. When used as a reflector system for a Jamain-type interferometer, the enables to interferometer to be used to measure relative movement of the reflectors.
In a first embodiment of the second aspect of the invention, the second reflector is a retroreflector. In this case, the second reflector preferably has three mutually-orthogonal reflective surfaces arranged around a second optical axis, in which case each of the reflective surfaces of the second reflector is preferably arranged parallel to a respective one of the reflective surfaces of the first reflector. The second reflector may comprise a second body of optical material which provides the reflective surfaces by internal reflection in the second body, and in this case the second body may have a transmitting surface for incident light which may be exactly parallel, but for some applications is preferably generally, but not exactly, parallel, to the second transmitting surface of the first reflector, so as to reduce the effects of stray reflections. Alternatively, the second reflector may comprise three plane mirror elements arranged around the second optical axis.
The first and second reflectors may be joined by a piezo electric material, which can be used to vary the spacing between the reflectors.
In a second embodiment of the second aspect of the invention, the second reflector comprises a plane mirror, the system further including a lens between the first and second reflectors.
In a third embodiment of the second aspect of the invention, the second reflector may comprise a polarising beam splitter, quarter-wave plate, plane mirror and second retroreflector arranged such that: light which is transmitted through the central region of the first reflector is transmitted through the beam splitter and quarter-wave retardation plate to the mirror; the light which is thus reflected by the mirror is transmitted through the quarter-wave plate, is reflected by the beam splitter, and is directed to the second retroreflector; the light which is thus reflected by the retroreflector is reflected by the beam splitter and transmitted through the quarter-wave plate to the mirror; and the light which is thus reflected by the mirror is transmitted through the quarter-wave plate, the beam splitter and the central region of the first reflector. Such an arrangement of the second reflector per se is known from FIG. 17 of US-A-5546184.
The apparatus of the third aspect of the invention is characterised in that: the retroreflector is arranged according to the first embodiment of the first aspect of the invention; and the first transmitting surface is generally, but not exactly, orthogonal to the optical axis of the apparatus, so as to reduce the effects of stray reflections.
The method of the fourth aspect of the invention is characterised by the step of cutting a cube corner retroreflector in a plane generally orthogonal to its optical axis, or forming a passageway through a solid cube corner generally in the direction of the optical axis of the cube corner.
The method of the fifth aspect of the invention is characterised by the steps of: providing three mirrors each having a pair of mutually orthogonal edges; cutting each mirror along a line intersecting the mutually orthogonal edges to form a first mirror and a second mirror; assembling the first mirrors mutually-orthogonally to form the first reflector.
The method may also include the step of assembling the second mirrors mutuallyorthogonally to form the second reflector.
The Jamin-type interferometer of the sixth aspect of the invention is characterised by a reflector system according to the second aspect of the invention. This therefore opens up a new range of applications for the Jamin-type interferometer, including the measurement of displacement of the two reflectors of the reflector system relative to each other, whilst retaining the advantages of the conventional Jamin-type interferometer and avoiding the disadvantages of the conventional Michelson-type interferometer.
The Jamin-type interferometer of the seventh aspect of the invention is characterised in that the reflector system is in accordance with the second aspect of the invention.
The Jamin-type interferometer of the eighth aspect of the invention is characterised in that: one of the projected beams is disposed between the return beams and/or one of the return beams is disposed between the projected beams. The enables a single phase-shifting means to be employed to produce a required phase shift in two of the beams.
Preferably, a first contiguous area of the face of the beam splitter is provided with a phase-shifting coating, one of the projected beams being projected from, and one of the return beams returning to, the first area. The masking which may be required for depositing of the coating is therefore simplified as compared with the case of US-A-5546184 wherein two separate areas of coating need to be applied.
Preferably, the coating produces a phase shift such that there is a phase difference of substantially xcfx80/2 radians between the two interferogram beams, and preferably the coating comprises a thin metal film.
Preferably, a second contiguous area of the face of the beam splitter is devoid of any coating providing any substantial phase-shift, the other projected beam being projected from, and the other return beam returning to, the second area.
Preferably, the interferometer employs a reflector system according to the second aspect of the invention.
The interferometers of the seventh and eighth aspects of the invention may be provided in combination with a dual-chamber gas or liquid refractometer arranged so that the first projected and return beams pass through one chamber of the refractometer and the second projected and return beams pass through the other chamber of the refractometer.
In the interferometers of the seventh and eighth aspects of the invention, the reflector system may comprise first and second retroreflectors rigidly joined together adjacent each other, the first retroreflector being arranged to retroreflect the first projected beam to produce the first return beam, and the second retroreflector being arranged to retroreflect the second projected beam to produce the second return beam. The interferogram(s) may therefore be used to measure changes in the inclination of the reflector system, so that an auto-collimator function can be provided.
The interferometers of the seventh and eighth aspects of the invention may further include means for modulating the optical path length of one of the beams, or one of the projected beams and its respective return beam. As will be described in more detail below, such modulation can be used in calibration of the detection of the interferogram beams. In a first embodiment of these aspects of the invention, the modulating means comprises an optically-transmitting, varying-thickness plate disposed in the path of the beam(s) to be modulated, and means for rotating the plate. In a second embodiment of these aspects of the invention, the modulating means comprises a layer of optically-transmitting, flexible material sandwiched between a pair of optically-transmitting plates disposed in the path of the beam(s) to be modulated, and means for modulating the spacing of the plates. In these cases, the modulating means may be disposed adjacent the beam splitter, for example as parts of a head unit of the interferometer.
In the case where the reflector system is provided by the first embodiment of the second aspect of the invention and the first and second reflectors are joined by a piezo electric material as described above, the modulating means preferably includes means for modulating the thickness of the piezo electric material to modulate the spacing between the reflectors.
The aim of the ninth aspect of the invention is to improve the reliability and/or accuracy of the resulting phase shift.
The method of the ninth aspect of the invention is characterised by the steps of: projecting a beam of light at the coating with an angle of incidence of substantially xcfx80/4 radians so that the beam is split into a transmitted component and a reflected component; monitoring the intensities or phases of the transmitted and reflected components during the baking step; and terminating the baking step when the monitored intensities or phases have a predetermined relationship.
It has been discovered that if the intensities of the reflected and transmitted components are equal, the phase shift is such as to produce a phase difference of xcfx80/2 radians between the interferogram beams. Therefore, for convenience, preferably it is the intensities which are monitored. The baking step may be terminated when the monitored intensities are generally equal. Alternatively, in the case where the intensity of the transmitted component is monitored after transmission through another surface of the member, the baking step is preferably terminated when the monitored intensity of the transmitted component is substantially equal to a predetermined proportion of the monitored intensity of the reflected component. This can therefore take account of the transmittance at the other surface of the block.
Preferably, the coating is of aluminium. Preferably, the baking temperature is between 450 and 480xc2x0 C. Preferably, the intensities of the polarisation components of the projected beam in the perpendicular and parallel directions relative to the plane of the incident light are generally equal to each other.
In accordance with the tenth aspect of the invention, there is provided a beam splitting block produced by a method of the ninth aspect of the invention.
In accordance with the eleventh aspect of the invention, there is provided an interferometer of the seventh or eighth aspect of the invention, wherein the beam splitter is provided by a beam splitting block of the tenth aspect of the invention.